A complex problem in surrogate surface design is determining the influence of the surface geometry on particle deposition. Other surface design parameters whose influence on particle deposition must be considered include surface roughness, surface electrical charge characteristics, and reactivity of the surface with particulate and gaseous species. In addition, the simplicity of extraction of deposited material for analysis is also of importance.
Based on the simplicity of its geometry, the smooth flat surface presents the fewest complications in modeling particle deposition. Furthermore, the problem of predicting the aerodynamic characteristics of a flat surface in wind tunnels has been studied extensively. Equations for such parameters as velocity profiles, boundary layer thicknesses, and particle deposition rates are well documented (Schlichting, H., "Boundary-Layer Theory"; McGraw-Hill, New York, 1979; Hinze, J. O., "Turbulence"; McGraw-Hill, New York, 1975; Sehmel, G. A., J. Colloid Interface Sci. 1971, 37, 891-906; Sehmel, G. A., Aerosol Sci., 1973, 4, 125-138; McCready, D. Ph.D. Thesis, Virginia Polytechnic and State University, Blacksburg, Va., 1984; and Lane, D. D., Stukel, J. J., J. Aerosol Sci., 1978, 9, 191-197). Nevertheless, when exposed to the ambient atmosphere, flat plates possess undesirable aerodynamic characteristics which seriously complicate particle deposition predictions.
Wind tunnel visualizations have been performed on a flat plate surrogate surface. Such a surface was constructed so that the deposition plate fit snugly into a recessed aluminum holder. Additionally, the plate holder had a 15.degree. knife edge. The plate and holder were designed to minimize the disturbance of the natural airflow patterns (Davidson, C. I., Lindberg, S. E., Schmidt, J. A., Cartwright, L. G., Landis, L. R., J. Geophys. Res., 1985, 90, 2123-2130).
At a 0.degree. angle of attack, the flow visualizations showed little disturbance of the natural airflow over the surface. However, even small positive changes (.ltoreq.2.degree.) in the angle of attack resulted in flow separation and extensive surface-induced turbulence at the leading edge of the plate. The presence of these flow disturbances introduce complexities in the calculation of the various aerodynamic parameters and the rate of particle deposition.